The present invention relates to a semiconductor switch circuit of field-drive type in which the main drive section is isolated electrically from its control section.
With the ever progressing electronic technologies in extensive fields of industry, there is an increasing demand for devices which are capable of controlling a large amount electric power with a small control signal, with electrical isolation being made between the control section and the main drive section of the device. Typical semiconductor devices that meet the demand are photo-coupling devices known as "photocouplers". Among such devices, photo-coupling thyristors having superior features such as: (1) forward and reverse blocking ability, (2) small power loss after switching, and (3) self-holding ability, are used extensively for switches of electronic exchange and solid state relays. The photo-coupling thyristor, however, has some crucial problems as will be described in the following.
FIG. 1 shows the basic arrangement of a switch circuit including a photo-coupling thyristor. With a switch 1 being closed, a current flows in a light emitting device 2, and it radiates the light. The light emission causes a photo-thyristor 3 to produce a photocurrent, and when the photo-thyristor is forwardly biased by an a.c. power source 4, it is turned on by the photocurrent. Reference numbers 5, 6 and 7 denote resistors. This arrangement, with the photo-thyristor and light emitting device isolated electrically, has the following advantages in contrast with the usual electrical coupling systems.
(a) There can be a voltage difference between terminals B and D in controlling the conducting operation. PA1 (b) The currents flowing in the light emitting component 2 and in the thyristor circuit are totally independent. PA1 (1) The photo-thyristor 3 and transistor 1 are made of silicon, while the light emitting device 2 is made of compound semiconductor of III-V group or II-VI group, typically GaAs. Due to different materials used, the device needs to be fabricated in a hybrid integrated circuit that requires precise assembling work, and therefore the cost is high. The fact that the processing technology of compound semiconductor wafers lagging behind that of silicon wafer technology also contributes to high cost. PA1 (2) Efficiencies are low in generating the light by the light emitting diode, transmitting the light to the photo-thyristor and receiving the light by the photo-thyristor. A low total photo-coupling efficiency requires a large control current of several milliamperes in the light emitting diode for activating the photo-thyristor.
On the other hand, the above arrangement has the following drawbacks.
Japanese Patent Examined Publications Nos. 42-24863 and 53-46589 disclose arrangements for activating a pnpn component by a MOS gate or MOS FET. Japanese Patent Unexamined Publication No. 57-196626 discloses an arrangement for turning on and off a pnpn component by a MOS FET. Although in any of the above cases the gate section is isolated from the main switch section, the main switch in a floating state cannot be turned on. Namely, the main switch is turned on only when the gate voltage is higher or lower than the cathode voltage of the main switch, and these arrangements do not function identically to the photocoupler.